We have proposed a seeding method using silica gel to remove silica from geothermal brines and applied to prevent silica scale on the heat exchanging surfaces.(1)The effects of the seed particle diameter, initial specific surface area and initial pore diameter on silica removal are experimentally examined using a model geothermal brine. The effects of the particle diameter on the quantity of the final precipitated silica and on the final specific surface area are hardly seen. The quantity of precipitated silica per unit surface area increases with increasing initial pore diameter, and the silica precipitation rate coefficient increases with decreasing particle diameter and with increasing initial pore diameter.(2)The effects of pH and some operational conditions on the silica removal performance are investigated. When the initial silica concentration is 0.5 kg-SiO2-m^<-3>, the silica removal ratio increases with increasing initial pH in the range of 5-9. When the initial pH is the same, the silica removal ratio increases with increasing temperature and with decreasing initial silica concentration. While the silica removal ratio is not affected by the pre-seeding time in the case of lower initial silica concentrations, it increases with decreasing pre-seeding time in the case of higher initial silica concentrations.(3)Model geothermal brines based on a component of dissolved salts in the actual geothermal brines were used, and the effect of dissolved salts on the silica removal performance by a seeding method using silica gel is examined. Potassium and sodium ions increase the silica supersaturation degree by their salt-out effects, and as a result promote silica removal. Calcium and ferric ions promote silica removal by production of their silicates in addition to the salt-out effect. Boric acid has little effect on silica removal.